1. Field of the Invention
The present invention relates to a power amplifier integrated circuit employed in wired or wireless system application, and more specifically, to a power amplifier integrated circuit having a distributed capacitor.
2. Description of the Prior Art
Power amplifier integrated circuits have been widely used in different kinds of wired or wireless system applications. Power amplifier integrated circuits such as those employing heterojunction bipolar transistors operate with elevated junction temperatures. High junction temperatures degrade device reliability and limit maximum current density, and thus maximum power, of the power amplifier.
When operated at high power, power amplifier integrated circuits can suffer thermal runaway where, because of emitter current non-uniformity or temperature profile non-uniformity, an emitter of the power amplifier integrated circuit conducts an increasing amount of current until catastrophic device failure. Operating at high power also reduces the device life and mean time to failure (MTTF). Moreover, a larger device is mandated for a given application when a power amplifier integrated circuit cannot operate at a required power.
Please refer to FIG. 1. FIG. 1 shows a prior art heterojunction bipolar transistor power amplifier 10. Typically, the power amplifier 10 includes a plurality of emitter fingers and base fingers. Accordingly, the power amplifier 10 is an equivalent circuit comprising transistors 12a-12c. The power amplifier 10 further includes ballasting resistors 14a-14c to stabilize the transistors 12a-12c when operating with a high current density, and bypass capacitors 16a-16c in parallel with the ballasting resistors 14a-14c. An RF input signal and DC voltage are supplied through a common input node 18 and output is taken from the collector nodes 20. The power amplifier 10 is further described by Khatibzadeh et al. in U.S. Pat. No. 5,321,279, which is included herein by reference. While, the ballasting resistors 14a-14c provide device stability, they also undesirably reduce the overall gain of the power amplifier 10.
FIG. 2 shows a second prior art power amplifier as described by Pratt in U.S. Pat. No. 5,629,648, which is included herein by reference. Similar to the power amplifier 10, the power amplifier 30 comprises transistors 32a-32c and ballasting resistors 34a-34c. Bypass capacitors 36a-36c are also provided. An RF signal can be applied at a node 38, a DC voltage can be applied at a node 39, and a corresponding output can be taken from collector nodes 40. However, use of capacitors 36a-36c is inefficient and mandates an overly complicated layout.
Generally, the prior art power amplifier circuits provide power amplifier stability at the cost of gain, and do so in an inefficient way. Moreover, layout of the prior art power amplifier integrated circuits is inefficient.
It is therefore a primary objective of the present invention to provide a power amplifier having a distributed capacitor to solve the problems of the prior art.
According to one preferred embodiment of the present invention, the power amplifier includes a plurality of transistors. Each transistor has a base and a ballast resistor having a first terminal and a second terminal. The embodiment further comprises a DC node, an RF node, and a capacitor having a third terminal and a fourth terminal. The first terminal of each ballast resistor is connected to a base of the plurality of bases, and the second terminal of each ballast resistor is connected to the DC node. The third terminal of the capacitor is connected to the RF node and the fourth terminal of the capacitor is connected to the plurality of bases of the transistor.
According to the preferred embodiment, the transistor is a heterojunction bipolar transistor.
According to the preferred embodiment, the ballast resistors are set to maximize uniformity of temperature of a plurality of emitters of the transistor.
According to the preferred embodiment, the capacitor has a substantially high capacitance so that an RF input signal applied at the RF node suffers low signal loss.
It is an advantage of the present invention that the capacitor provides a distinct path for the RF input signal, such that the RF input signal does not suffer significant signal loss.
It is a further advantage of the present invention that the capacitor supplies capacitance that is distributed to each base of the plurality of bases.